The present invention generally relates to the field of chemical mechanical polishing. In particular, the present invention is directed to a chemical mechanical polishing pad having a groove network designed to optimize polishing medium residence time across the article being polished.
In the fabrication of integrated circuits and other electronic devices, multiple layers of conducting, semiconducting and dielectric materials are deposited onto and etched from a surface of a semiconductor wafer. Thin layers of conducting, semiconducting and dielectric materials may be deposited by a number of deposition techniques. Common deposition techniques in modern wafer processing include physical vapor deposition (PVD), also known as sputtering, chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD) and electrochemical plating. Common etching techniques include wet and dry isotropic and anisotropic etching, among others.
As layers of materials are sequentially deposited and etched, the uppermost surface of the wafer becomes non-planar. Because subsequent semiconductor processing (e.g., photolithography) requires the wafer to have a flat surface, the wafer needs to be planarized. Planarization is useful for removing undesired surface topography as well as surface defects, such as rough surfaces, agglomerated materials, crystal lattice damage, scratches and contaminated layers or materials.
Chemical mechanical planarization, or chemical mechanical polishing (CMP), is a common technique used to planarize workpieces, such as semiconductor wafers. In conventional CMP using a dual-axis rotary polisher, a wafer carrier, or polishing head, is mounted on a carrier assembly. The polishing head holds the wafer and positions the wafer in contact with a polishing layer of a polishing pad within the polisher. The polishing pad has a diameter greater than twice the diameter of the wafer being planarized. During polishing, each of the polishing pad and wafer is rotated about its concentric center while the wafer is engaged with the polishing layer. The rotational axis of the wafer is generally offset relative to the rotational axis of the polishing pad by a distance greater than the radius of the wafer such that the rotation of the pad sweeps out a ring-shaped “wafer track” on the polishing layer of the pad. The radial distance between radially inner and outer boundaries of the wafer track defines the width of the wafer track. This width is typically equal to the diameter of the wafer when the only movement of the wafer is rotational. The carrier assembly provides a controllable pressure between the wafer and polishing pad. During polishing, a fresh polishing medium, e.g., polishing medium, is dispensed close to the rotational axis of the pad within the inner boundary of the wafer track. The polishing medium enters the wafer track from the inner boundary, flows into the gap between the wafer and the pad, contacts the wafer surface, and exits the wafer track at its outer boundary close to the edge of the pad. This movement of the polishing medium occurs in a substantially radially outward direction due to the centrifugal force induced on the polishing medium as a consequence of rotation of the pad. The wafer surface is polished and made planar by chemical and mechanical action of the polishing layer and polishing medium on the surface.
In a typical CMP process involving the use of reactants in the polishing medium, when the polishing medium is exposed to the wafer surface within the wafer track of the pad, the reactants interact with features on the wafer being polished, e.g., copper metallurgy, thereby forming reaction products. As the dispensed polishing medium flows from the inner boundary to the outer boundary of the wafer track, the residence time for the polishing medium under the wafer surface increases. Interaction of the polishing medium with the wafer material causes a variation in relative proportions of the reactants and reaction products in the polishing medium, as measured along a radius of the pad. The polishing medium near the inner boundary of the wafer track has a relatively high proportion of reactants (much like fresh polishing medium), and the polishing medium near the outer boundary of the wafer track has a relatively low proportion of reactants and a relatively high proportion of reaction products (much like spent polishing medium).
Polishing reaction rates in general may depend differently on the concentrations of reactants and products in the polishing medium. Hence polishing at any given location on the wafer is influenced by the relative proportions of reactants and reaction products in the polishing medium exposed to. Furthermore, an increase in the relative amount of reaction product at a given location will typically either increase or decrease the polishing rate at that location, all other factors being equal. To achieve the polishing rates across the entire wafer necessary to obtain a planar surface, it is not enough to merely control the quantity of polishing medium available to the wafer at a given radial location. Instead all locations on the wafer should be uniformly exposed to polishing medium containing different concentration levels of reactants and the reaction products. Unfortunately, known CMP systems and associated polishing pads do not typically distribute polishing medium in such a manner.
It is known to provide outward extending grooves in a polishing pad that have decreasing depth so as to slow the radial flow rate of slurry applied to the pad. Such a groove pattern is described in U.S. Pat. No. 5,645,469 to Burke et al. While the groove pattern described in the '469 patent may slow the radial flow rate of slurry to some extent, it does so using straight, radially extending grooves, the depth of which begin decreasing at an equal radial distance from the center axis of the pad.